In general, an organic EL element includes a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL element uses the phenomenon that, when an electric field is applied between both the electrodes, electrons are injected from a cathode and holes are injected from an anode, and each electron and each hole recombine in the light-emitting layer to emit light as energy.
In recent years, progress has been made in developing an organic EL element using an organic thin film. In particular, development has been made to enhance luminous efficiency. In the course of the development, the efficiency of injection of carriers from the electrodes has been improved through the optimization of the kind of electrodes. In addition, there has been developed an element in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer-cum-electron-transporting layer formed of an 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are used, resulting in a significant improvement in luminous efficiency, as compared to related-art elements. Thus, the development of the organic EL element has been promoted with a view to accomplishing its practical application to a high-performance flat panel having features such as self-luminescence and rapid response.
Investigations have also been made on using a phosphorescent light-emitting material rather than a fluorescent light-emitting material as an attempt to raise the luminous efficiency of an element. Many kinds of elements including the element in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of Alq3 are formed use fluorescent light emission. However, by using phosphorescent light emission, that is, by using light emission from a triplet excited state, luminous efficiency is expected to be improved by about three times to four times, as compared to the case of using related-art elements in which fluorescent light (light emission from a singlet excited state) is used. In order to accomplish this purpose, investigations have been made on adopting a coumarin derivative or a benzophenone derivative as a light-emitting layer, but extremely low luminance has only been provided. After that, investigations have been made on using a europium complex as an attempt to use a triplet excited state, but highly efficient light emission has not been accomplished. Among the investigations involving using phosphorescent light emission, many investigations on a phosphorescent light-emitting dopant centered on an organometallic complex such as an iridium complex have been made, and ones capable of highly efficient light emission have been found.